1. Field Of The Invention
This invention relates to boom carrying cranes which are usually mounted upon a mobile type of wheeled platform or truck bed for rotation about a fixed axis extending vertically through said platform or truck bed.
2. Description Of The Prior Art
In recent decades, increasing use has been made at demolition and construction sites of large mobile cranes which may be mounted upon a truck bed or other type of wheeled platform. These cranes may include a turret capped by a cab in which the operator may be seated, and from which turret and/or cab there may extend upwardly at an angle a crane boom supported at a preselected and changeable angle by a plurality of cables, usually extending from the turret, cab or another boom, to one or more points on the main boom. Such a crane is driven or rolled into a selected position at a building demolition or construction site where it may be secured against undesired lateral movement by a drop-down leg or other fixed device. Since, in many uses of such cranes, the boom may be operated either near high tension wires posing a great hazard to the crane operator, or near buildings or other structures which it is not desirable to have the crane boom strike in the course of its use at the site, a number of different means have been devised to prevent the crane operator from inadvertently allowing his crane boom to be swung too close to any proximate power lines or structures.
Among such prior art means have been disclosed in the following United States Pat. Nos.
R. g. ely -- 3,456,810 PA1 T. w. thomas -- 3,447,692 PA1 Orendorff -- 3,664,515.
In these prior art patents the approach has generally been to provide a power cutoff for the driving means with or without some type of stop member which serves to prevent further rotation of the turret. Interposing any type of stop member, however, against which the heavy revolving turret will strike in order to block further rotation can result in producing a considerable jarring of the cab to the discomfort of the operator and, moreover, can result in damage to the parts of the crane which are involved with the turret rotation and its stoppage. In addition, where the effort has been to shut down the drive means by which turret rotation is effected, it is usually necessary to have the limiting means built into the crane turret driving system at the factory at the time the crane is being constructed. Such a system, therefore, does not readily lend itself to being later adapted to cranes previously built and which have been in operation without any such drive cutoff and/or blocking system.
The principal method which has been utilized to prevent crane booms from overswinging to strike power lines or other structures where the cranes have not been equipped with any prior art systems of the types disclosed in the patents cited above, has been to provide a heavy cable to extend laterally from the upper portion of the crane boom in a direction opposite from the power lines or other structures which are not to be struck, and to anchor this cable on a caterpillar tractor of sufficient size that it will effectively limit the lateral movement of the crane boom toward the power lines or other structures when the cable connecting the boom and the tractor is made taut. While this method of limiting the swing of the crane boom may be effective in some situations, it is wastefully expensive in that it ties up the construction company's tractor and prevents it from being otherwise employed at the building or demolition site, and further requires the presence of a high-priced tractor operator on the tractor itself. Moreover, in the case of the very large cranes with huge booms, there may be very few, if any, tractors which are of sufficient weight to stop the heavy boom from over swinging; i.e., the boom may be of such a total weight that if it acquires any inertia, it may actually pull the tractor from its stopped position toward the power lines or structures which are to be avoided.
The prior art has not, therefore, provided effective means for preventing crane boom overswings, particularly with respect to older existing cranes into which limiting systems of the prior art patents have not been built into their respective driving means.